nix flake prefetch-inputs: Skip build-time inputs

src/nix/flake-prefetch-inputs.cc

@@ -44,6 +44,8 @@ struct CmdFlakePrefetchInputs : FlakeCommand
                 return;
 
             if (auto lockedNode = dynamic_cast<const LockedNode *>(&node)) {
+                if (lockedNode->buildTime)
+                    return;
                 try {
                     Activity act(*logger, lvlInfo, actUnknown, fmt("fetching '%s'", lockedNode->lockedRef));
                     auto accessor = lockedNode->lockedRef.input.getAccessor(store).first;

src/nix/flake-prefetch-inputs.md

@@ -12,6 +12,6 @@ R""(
 
 Fetch the inputs of a flake. This ensures that they are already available for any subsequent evaluation of the flake.
 
-This operation is recursive: it will fetch not just the direct inputs of the top-level flake, but also transitive inputs.
+This operation is recursive: it fetches not just the direct inputs of the top-level flake, but also transitive inputs. It skips build-time inputs, i.e. inputs that have the attribute `buildTime = true`.
 
 )""

tests/functional/flakes/build-time-flake-inputs.sh

@@ -61,3 +61,28 @@ rm -rf "$lazy"
 clearStore
 
 expectStderr 1 nix build --out-link "$TEST_ROOT/result" -L "$repo" | grepQuiet "Cannot build.*source.drv"
+
+# `nix flake prefetch-inputs` should ignore build-time inputs.
+depDir=$TEST_ROOT/dep
+createGitRepo "$depDir"
+createSimpleGitFlake "$depDir"
+
+cat > "$repo/flake.nix" <<EOF
+{
+  inputs.lazy = {
+    type = "git";
+    url = "file://$depDir";
+    buildTime = true;
+  };
+
+  outputs = { self, ... }: { };
+}
+EOF
+
+nix flake lock "$repo"
+
+clearStore
+rm -rf "$TEST_HOME/.cache"
+rm -rf "$depDir"
+
+nix flake prefetch-inputs "$repo"